Remotely located safety switches and "dead-man's" switches have been used on a variety of tools and machines for the purpose of shutting off such tools and machines in the event of an accident or incapacitation of the equipment operator. Typical "dead-man's" switches include throttle controls in the form of handgrips which must be squeezed during operation of the throttle. In the event the operator becomes incapacitated, the squeezing ceases; and the throttle no longer is operated. Other types of secondary controls for effecting a similar purpose also have been developed and generally operate in parallel with or in conjunction with a normal on/off switch control or a throttle control to turn off the machine or turn off the throttle in the event of a malfunction or incapacitation of the operator.
Another type of safety switch is in the form of a foot operated switch. Typical switches of the type are disclosed in the patents to Ruben U.S. Pat. No. 2,944,120; Nicholas U.S. Pat. No. 2,313,166; and Child U.S. Pat. No. 2,111,311. The patents to Child and Nicholas simply are directed to foot operated power switches which are spring biased to be normally "off". These switches are substantially standard single-pole, double-throw electrical switches and are used as the main power switch to turn on or to turn off the power to the tool or the device to which they are connected.
The device of the patent to Ruben is similar to the devices of Child and Nicholas; but Ruben includes an extra safety feature, inasmuch as the system of Ruben requires a first depression and release of the switch to set the circuit for operation followed by a subsequent depression of the switch to initiate and continue operation. Upon a second release of the switch, the system is turned off. This is done to prevent accidental start-up from the "off" status of the system controlled by the switch. The foot switch of Ruben itself, however, is the primary on/off switch for the system; and this switch, as in Child and Nicholas, is used to control turning on of the system as well as turning the system off.
The patent to Khatri U.S. Pat. No. 4,535,254 discloses a touch-operated power control switch device with indicators. While this device electronically is considerably more sophisticated than the systems of Ruben, Nicholas and Child, Khatri also basically acts as a simple on/off switching system for controlling the turning on and turning off of power to a load.
Another patent directed to a press machine with a foot operated switch control is the patent to Richards U.S. Pat. No. 4,283,612. In Richards, the switch functions as a "dead-man's" switch where the foot switch must be continuously depressed in order to operate the press. Additional proximity switches are employed to keep the operator at a safe distance from the tool. If the foot operated switch is not depressed, the tool is turned off. Subsequent depression of the foot operated switch, without changing any of the other controls or switch settings, reactivates or turns back on the power for the press.
The patent to Meyerhoefer U.S. Pat. No. 4,359,615 discloses a safety interlock electro-mechanical switch in the form of a "key" to prevent unauthorized operation of a tool or electrical device such as a table saw. The tool cannot be turned on until the special key is inserted into an actuator on the switch to permit the operation of the power switch. With the key in place, however, the power saw or other tool of Meyerhoefer is operated in a conventional manner and does not include any safety switch interlock or "dead-man's" switch to turn it off in the event of a tool malfunction or incapacitation of the operator.
It is desirable to provide a safety switch for electrical devices such as power tools and machines of all types which quickly and reliably turns off the tool or machine from a remote location irrespective of the location of the main on/off power switch for the tool or machine. It further is desirable to provide a remote emergency cut-off switch which operates as an "off-only" safety device which cannot be used to turn back on the tool or machine it controls until it is reset by turning off the tool and then turning the tool back on by means of the conventional main power supply switch.